Method of and apparatus for testing acoustic translating devices



Det. 3l, 1,939. v R. G. PIETY 2,178,347 'METHOD oF .um Armmus Fn TESTING Acounc'TRANsLATING nEvIcEs Filed Sept. 11. 193v Suunto:

Patented oet. 31, 1939 UNITED STATES METHOD F AND APPARATUS FOR TESTING ACOUSTIC TRANSLATING DEVICES Raymond G, Piety, Bartlesville, Okla., assigner' to RadioA Corporation of America', a corporation oi Delaware Application sepeembeeii, i937, serial No. 169,434"

This invention relates to a method of and apparatus for testing such acoustic translating devices as loudspeakers, microphones, and otherAv v analogous devices. for transforming acoustic' energy into electrical energy, or vice versa.

Theinvention involves a novel apparatus forand method of operation thereof whereby the output from a loudspeaker or microphone vis readily and directly measured in terms of amplitude, independent of frequency, over the entire' frequency range. This is accomplished through the use of a pair of fixed oscillators having a predetermined beat frequency and a variable oscillator which is swept overthe frequency range of the apparatus, the output from the device being electrically converted into a current of the beat frequency between the two xed oscillators, which is then measured directly for its amplitude. The speed of sweep over the `frequency range is-so chosen as to avoid any reverberation effects.

One object of my invention is to provide an improved apparatus for testing'acoustic translating devices.

Another object of my invention is to provide an' .35 improved method of measuring the amplitude of response of acoustic translating devices.

Another object of my invention is to provide an improved method of measuring the response of acoustic translatingdevices over a wide range y a0 of frequencyin terms of a single beat frequency. Another object of my invention is to provide an improved method of measuring the response of acoustic translating devices which will avoid ,the eilect of reverberation. 'f'

v35 Other and incidental objects of my invention. will be apparent to those skilled in the art from a reading of the following specicatlon and an inspection of the .accompanying drawing in which The single Igure of the drawing is a schematic n diagram lof thenpreferred form of my invention.

As shown in the drawing, the apparatusinvolves two xed'oscillators I and 2. These `oscillators operate at any convenient frequency above -audio frequency and have a frequency difference from the xed oscillator I, producing audio` frequency beat notes, which, in turn, are trans- (i.i79-175) mitted to the apparatus 5, indicated as a loudspeaker, which, is under test. The sounds produced bythe loudspeaker` 5 are picked up by the microphone 6, the electrical impulses from which are amplified by the amplifier 'I and are fed to the 5 transformer 8. f

Another portion of the output from the variable oscillator 3`is passed to the demodulator and amplifier 9, Where it is mixed Awith the output from the fixed oscillator 2 producing a beat note l0 differing from the beat noteI applied to the apparatus 5 by the diiference between the oscillators I and 2, irrespective ofthe frequency of the variable oscillator 3. The output from this demodulator and amplifier 9 is fed to the trans- -l former IIJ, which hasa balanced secondary feeding the grids of the tubes Il and I2. This arrangement, involving the balanced transformer III, the transformer 8 and the tubes II and I2,

forms a balancd'demodulator mixing the outputs 20 from the amplifier 1 and from the demodulator 9 to produce a beat frequency corresponding to the frequency difference between the oscillators I and 2, i. e., 40 cycles, and having an amplitude corresponding to the amplitude of the sounds picked up by the microphone 6. The output from the tubes II and I2 isfedthrough the condensers I3 and I4 to the amplier I5, which is tuned by Athe condenser I6 and inductance I 'I to the beat frequency between the oscillators I and 2, which 80 is also thefrequency of the output from the tubes Il and I2, The output circuit of the tubes II and I2 is tuned, like that of the tube I5, to this difference frequency. 'I'he output from the tube I5 is fed to the second amplifier tube I9, where it is further amplied, and passed through the transformer 20 to the meter 2l.A The meter 2| reads the amplitude of the output from the tube I9, which corresponds to the amplitude of the beat note impressed on the tubes. I I and I2, and which o therefore corresponds to the output of the device 5 under test. v In the operation of the device, I find it highly advantageous, in order to avoid the effects of reverberation in the room in whichthe'apparatus 45 is used, to move the variable oscillator 3 over the desired range at such a velocity that by the time any refiected'sound can reach the microphone 6 thel device will be operating on a frequency suficiently different from its previous frequency so 50 that the beat frequency due to the reverberation will be filtered .out in the balanced amplifier above described which, as above stated, is tuned to a predetermined frequency, such as cycles.

It will be-apparent that the use of this apu speakersbutthatifacalibratedloudspeakeris usedat I,theapparatusmaybeusedfortesting micrhones. Likewise, by the me of appropriate mechanical connections or acoustic couplins'. @he

' apparatus may b eused for testing such devices as telephonereceivers, transmitters or',l

phonograph pick-ups.

Having now described' my invention, I claim: l. In combination, means for simultaneously producing two sets of oscillations diii'ering by a predetermined frequency, means for transmitting one of said sets of oscillations through the apparatus under test. means vior mixing the two sets of oscillations, and means for measuring the amplitude of the oscillation of the difference irequency. l

s. Apparatus of the uw described including' of the apparatus under test and said second beat note, and means for measuring the amplitude of the beat note thereby produced.

3. The method of testing electrical apparatus comprising simultaneously producing two sets of oscillations differing by a predetermined frequency, transmitting one ot said sets of oscillations through the. apparatus under test. mixingthe two sets of' oscillations,- and measuring the amplitude of the oscillation of the dierence frequency.

4. The method oi testing electrical apparatus comprising simultaneously producing two sets oi' oscillations differing by a, predetermined frequency, transmitting one of said sets of oscillations through the apparatus under .`test, mixing the two sets`of oscillations to produce a beatffre# arrasar Aannum une: nmited'm meinung of 1mmquency, and measuring cilia'tions oi the beat frequency.

theampliiauieottheos-y 5. In testing'of' acoustic apparatus, tbs meth-I' od of avoiding reverberation effects, including the steps of lproducing soimd waves for impression ot said acoustic apparatus, varying the frequency of said sound waves at a speed greater than the speed of reverberation, and adjusting said apparatus to reoeiveonly the frequencies of said waves received directly from the producer of said waves.

6.1'hemethodoftestingelectricalapparatus comprising simultaneously producing two sets of oscillations diiIering by a predetermined frequency. transmitting one of said sets of oscillations through the apparatus imder test. mixing the two sets of oscillations. measuring the ampiitude of the oscillations oi! the difference frequency, and changing the frequency of said oscillations at such a rate that any difference frequency due to reverberation will ,be diilerent from f the frequency diiference of said oscillations.

7. In testing of acoustic apparatus, the method' reilection oi a previously produced wave impinges on said apparatus, and' tuning' said apparatus to the frequency of said directly impressed sound ent frequency is being produced at the time s.V

` waves to filter. out the frequency of said waves.

8. A testing system for acoustic apparatus comprising means forgenerating sound waves having a continuous variable frequency, said generating means varying the ,frequency of said waves at a rate such that the frequency being impressed directly fromsaid generator is diilerent from. the frequency of the reflected waves reaching said acoustic apparatus, and means for tuning .said apparatus to the frequency oisaid directly impressed wave, said tuning means nltering out the frequencyof said reflected waves.

RAYMOND G. PIETY. 

